The dual slope type analog-to-digital slope converter for effecting a digital output signal from a measured analog input signal is well known as disclosed, for example, in U.S. Pat. Nos. 3,061,939; 3,316,546; 3,458,803, 3,660,834 and 3,566,397. Briefly, the method of conversion involves integrating a current directly related to an unknown voltage for a fixed period of time, followed by the integration of a standard current related to a reference voltage of opposite polarity until the integrator output returns to zero. The total time period required to null the integrator is directly proportional to the ratio of the measured current to the standard current, and, therefore, to the measured voltage. The integrator, therefore, is a circuit producing a linearly changing output with time (usually a ramp) when the input is some constant voltage and the rate of integrator output voltage increase is directly proportional to the magnitude of input voltage. When input voltage is zero, output voltage is not subject to change but remains zero at whatever output value was achieved at the beginning of the time period.
Standard operation for such prior art converters has included integration of the unknown in the same direction of polarity as the input signal, e.g. positive-to-positive which is then switched to a reference signal of opposite polarity that is integrated to zero. This is then detected by a comparator of the integrated signals and for large analog inputs prolonged time periods are required to effect the zero integration. The digital counts are then accumulated in a register proportional to the time factor associated with the unknown integration.
While this basic arrangement has functioned well with a high order of accuracy, it requires reference switching and polarity detection which becomes difficult at very low inputs leading to switching uncertainties. Also, bias currents associated with these prior devices have added and subtracted from the slopes in each of its changed directions. For overcoming these deficiences, it has been necessary to utilize precision low offset amplifiers. Despite recognition of the foregoing, means for effecting their elimination has not heretofore been known.